This is a very common problem with gas guns. You have brass, fired in a different rifle, sized by a standard sizing die, and it is oversize for your new rifle.

I am assuming that you are using a standard sizing die, and that you did not set up the die with a cartridge headspace gage.

I have a bunch of Wilson cartridge headspace gages. They cost $15.00 or so. There are two ledges on the back. Each ledge corresponds exactly with “GO” and “No Go” chamber headspace gages. The deeper ledge is the “Go”, the upper ledge is the “No Go”.

Depending on the resolution of your screen, these ledges might be hard to see in the attached picture. The difference between "Go" and "No go" is only around .006".

You set up your sizing die by simply sizing a case then dropping it in the gage. If the shoulder to base length of the case is above the “No go”, you turn the die in a little more to size the case a little more. If the case base is below the “Go”, you are oversizing your case and need to back the die out a bit.

I recommend to size to gage minimum for gas guns. Bolt guns are less fussy with ammo so you can play around with shoulder depth to extend case life.

There are more sophisticated gages on the market and many bolt gun users set the case shoulder back an exact amount, like .003” or so.

Wilson gages are cut with a special reamer: the distance from shoulder to base is correct but the gage is cut big between the shoulder and the base. You can drop a fired case into one of these gages and determine from the expanded case if you have a headspace issue with your rifle chamber. Plus or minus a little error.

There have been a few times when I found I could not size a case below the "No Go" of the gage no matter how much I turned the sizing die down. That is when I figured out that the die was too long and I had to grind material from the bottom of the die to get the sized cases to the proper length.

You will occasionally read threads where this happens to other folks. They completely full length size a case and can't figure out why they are having stiff bolt closure. Without gages they did not know they had a sizing die problem.

You will also read threads where reloaders claim that small base dies "over work" the brass. Mostly likely they are oversizing their cases in the small base dies and don't know what they are doing because they don't have a gage.

I use small base dies for all of my gas gun calibers: 223, 308 and 30-06. I would use them for other calibers if they were available. It is highly likely that your once fired brass is too fat. Even if you size the stuff to the correct length, you need a small base die to size it to fit in your chamber.